The present invention relates to a method and apparatus for injection molding, and in particular, to a method and apparatus for molding injection molded parts.
Conventionally, a variety of methods have been utilized for injection molding in various fields. Of these methods, a molding metal mold of a runnerless (hot runner) system has been widely used.
There are a variety of molds in such a hot runner system. Smaller molds generally only require one inlet for injecting molten material. For larger molds, several inlets are used to inject molten material at different points within the mold cavity. These larger molds are sometimes referred to as multi-gate mold cavities. In multi-gate mold cavities, the pressure of molten material differs at various points inside the cavity. The pressure typically becomes constant throughout the cavity once the cavity is completely filled with molten material.
A conventional molding process can be done using power from hydraulic means or electrical means. The molding process uses two platens, a movable platen and a stationary platen. In a process using hydraulic means, a hydraulic cylinder applies a certain force to push a movable platen against a stationary platen. Molding members within or attached to the platens form a molding cavity. The force is maintained on the stationary platen or die plate while a molten material is injected into the molding cavity. The molten material is injected into the cavity with a resin feeding screw until the pressure inside the cavity reaches a predetermined molding pressure or until the screw has moved a predetermined distance and for a set period of time, thereby ensuring that the cavity is filled. After injecting the molten material, the molten material is allowed to cool and solidify, the force is then released, and the plates are separated and the process begins anew.
Some injection molding machines use a mold with only one cavity, thereby allowing for the production of one molded object per cycle. Total cycle time is the sum of the fill time and the cool down time. The cool down time is generally substantially longer than the fill time. For example, a typical fill time is about 5 seconds, whereas a typical cooling time is about 30 seconds, for a total of about 35 seconds for the production of one molded article.
To reduce process time per molded article, some injection molding machines utilize molds with a plurality of cavities for forming a plurality of molded articles. The molten material fills into each of the cavities simultaneously. While this may extend the fill time a few seconds, for example for mold cavities for car doors, to about 8 seconds, the cooling time remains fixed at about 40 seconds. The total time of this process is about 48 seconds for the production of two molded articles. Thus, using multiple cavities increases the efficiency almost two-fold.
A problem with the multiple cavity method, however, is that the mold clamping force must also be doubled since the article molding area is doubled. As a result, a larger injection molding machine must be used to apply the extra force needed to hold the molding platens together. A larger injection molding machine costs more, takes up more floor space, and requires more power. Therefore, using multiple cavity molds with the conventional method can sacrifice cost for greater time efficiency.
Furthermore, for molding larger articles, the molten material is injected at several different points in the mold cavity. This is due to the limits on the flow of molten plastic. These larger mold cavities are commonly known as multi-gate mold cavities. An example of an article that would require a multi-gate mold cavity is an interior car door panel, which typically requires four or five gates per single cavity mold. In the manufacture of such parts, it is desirable to maintain the injected pressure of the molten material constant so that the part is formed accurately. Without maintaining the pressure constant, the structural accuracy of the formed part may suffer. For example, the resulting part may include short shots, ripples, or other dimensional inaccuracies. As such, there is a need to be able to accurately measure the pressure of molten plastic inside of the mold cavity.
Accordingly, a general object of the present invention is to provide an injection molding machine and a method for injection molding either large or small articles where there is process control for each cavity.
A further object of the present invention is to provide an injection molding machine and a method for injection molding large articles with greater efficiency and reduced costs.